Up to 2/3 of youth with autism spectrum disorder (ASD) exhibit challenging and often dangerous behaviors, including aggression and self-injury, that profoundly limits their access to community, educational, and therapeutic resources. Risk for challenging behavior and its consequences (e.g., psychiatric hospitalization; polypharmacy) increases during adolescence, particularly for severely affected youth with ASD (SA-ASD; characterized by intellectual disability or minimal verbal ability). Identifying contributors to challenging behaviors that can be modified using non-pharmacological and non-invasive approaches is a high priority for clinicians, researchers, and family caregivers. Yet, SA-ASD youth are under-represented in research, which restricts the potential for improvements for those who need it most. Sleep and the circadian regulation of sleep are modifiable through behavioral and chronotherapeutic interventions. In typically developing youth, the homeostatic and circadian regulation of sleep exhibits marked changes during adolescence (e.g., circadian phase delay). These changes confer risk for sleep problems and challenging behaviors mediated in part by changes in alertness, mood, and cognition. The proposed cross-sectional mentored study (K99) will evaluate if circadian phase delay is associated with sleep problems and challenging behaviors in SA-ASD youth. The study will be conducted in an inpatient psychiatric unit to facilitate training with, and enrollment of, SA-ASD youth, a population that is difficult to study in the community. The study will serve as a platform for training in (1) supportive techniques and non-invasive sleep measurement methods to increase participation of SA-ASD youth in objective sleep and circadian science, (2) systematic coding of observed challenging behavior and ecological momentary assessment methodology, (3) advanced analytic methods to model prospective associations and individual differences, and (4) the process of intervention development to support future intervention research. Training and lessons learned during the mentored phase will inform and support an independent prospective study (R00) of associations between circadian phase, sleep, and challenging behaviors in an independent sample of SA-ASD adolescents. The proposed study will follow SA-ASD youth as they are discharged from a psychiatric inpatient facility and return home. Following a pre-discharge baseline assessment in the hospital, in-home assessments of circadian phase and sleep and ecological momentary assessments of challenging behavior will be conducted at 6 and 12 months post-discharge. The objective of this study is to determine if circadian phase delay precedes the emergence/escalation of sleep problems and challenging behavior. This work will set the foundation for future studies to compare prospective associations between circadian phase, sleep, and challenging behavior across multiple comparison groups (e.g., higher- functioning ASD and typical development) and evaluate the clinical utility of intervening upon circadian phase to improve sleep and challenging behavior in ASD youth.